1. Field of Invention
The present invention pertains to the field of magnetic memories. More particularly, this invention relates to a magnetic memory structure that yields improved half-select margins.
2. Art Background
A magnetic memory such as a magnetic random access memory (MRAM) typically includes an array of magnetic memory cells. A typical magnetic memory cell includes a layer of magnetic film in which magnetization is alterable and a layer of magnetic film in which magnetization is fixed or "pinned" in a particular direction. The magnetic film having alterable magnetization may be referred to as a data storage layer and the magnetic film which is pinned may be referred to as a reference layer.
Typically, the logic state of a magnetic memory cell is indicated by its resistance which depends on the relative orientations of magnetization in its data storage and reference layers. A magnetic memory cell is typically in a low resistance state if the orientation of magnetization in its data storage layer is substantially parallel to the orientation of magnetization in its reference layer. A magnetic memory cell is typically in a high resistance state if the orientation of magnetization in its data storage layer is substantially anti-parallel to the orientation of magnetization in its reference layer.
A magnetic memory cell is usually written to a desired logic state by applying external magnetic fields that rotate the orientation of magnetization in its data storage layer. Typically, the orientation of magnetization in the data storage layer aligns along an axis of the data storage layer that is commonly referred to as its easy axis. Typically, the external magnetic fields are applied to flip the orientation of magnetization in the data storage layer along its easy axis to either a parallel or anti-parallel orientation with respect to the orientation of magnetization in the reference layer depending on the desired logic state.
Prior magnetic memories usually include an array of word lines and bit lines which are used to apply external magnetic fields to the magnetic memory cells during writing. The magnetic memory cells are usually located at intersections of the word lines and bit lines.
A selected magnetic memory cell is usually written by applying electrical currents to the particular word and bit lines that intersect at the selected magnetic memory cell. Typically, an electrical current applied to the particular bit line generates a magnetic field substantially aligned along the easy axis of the selected magnetic memory cell. The magnetic field aligned to the easy axis may be referred to as a longitudinal write field. An electrical current applied to the particular word line usually generates a magnetic field substantially perpendicular to the easy axis of the selected magnetic memory cell.
Typically, only the selected magnetic memory cell receives both the longitudinal and the perpendicular write fields. Other magnetic memory cells coupled to the particular word line usually receive only the perpendicular write field. Other magnetic memory cells coupled to the particular bit line usually receive only the longitudinal write field.
The magnitudes of the longitudinal and the perpendicular write fields are usually chosen to be high enough so that the selected magnetic memory cell switches its logic state but low enough so that the other magnetic memory cells which are subject to either the longitudinal or the perpendicular write field do not switch. An undesirable switching of a magnetic memory cell that receives only the longitudinal or the perpendicular write field is commonly referred to as half-select switching.
Manufacturing variation among the magnetic memory cells typically increases the likelihood of half-select switching. For example, manufacturing variation in the longitudinal or perpendicular dimensions or shapes of the magnetic memory cells may increase the likelihood of half-select switching. In addition, variation in the thicknesses or the crystalline anisotropy of data storage layers may increase the likelihood of half-select switching. Unfortunately, such manufacturing variation decreases the yield in manufacturing processes for magnetic memories and reduces the reliability of prior magnetic memories.